1. Field of the Invention
The invention relates generally to the field of electromagnetic measurements of rock formation resistivity made by instruments disposed in wellbores drilled through rock formations. More specifically, the invention relates to methods for determining resistivity, resistivity anisotropy and formation attitude (dip) using electromagnetic measurements of the rock formations.
2. Background Art
One of the major recent developments in well logging technology is the introduction of electromagnetic measurements with three dimensional (3D) sensitivities. In so called “wireline” measuring systems (i.e., those conveyed through wellbores at the end of an armored electrical cable), 3D electromagnetic induction measurements are designed primarily for detecting resistivity anisotropy in vertical wells (see, e.g., Krieghauser et al, A New Multicomponent Induction Tool to Resolve Anisotropic Formation, paper D presented at the 2000 41st Annual SPWLA Symposium, Salt Lake City, Utah, 30 May-3 June, and Rosthal, R., Barber, T., Bonner, S., Chen, K. C., Davydycheva, S., Hazen, G., Homan, D., Kibbe, S., Minerbo, G., Schlein, R., Villegas, Wang, W., and Zhou, Field tests of an experimental fully triaxial induction tool, presented at 2003 SPWLA Annual Logging Symposium, June 22-25, Galveston, Tex., paper QQ.
Logging-while-drilling (“LWD”) measurements made by well logging instruments such as one identified by the trademark PERISCOPE, which is a trademark of the assignee of the present invention, represent an LWD counterpart of multi-axial wireline induction instruments. LWD instruments are typically conveyed through wellbores during drilling or during “tripping” of drill pipe or other pipe through a wellbore. The foregoing PERISCOPE LWD instruments are typically used for well placement within selected subsurface rock formations or reservoir portions of such formations. However the full potential of these multi-axial LWD measurements for quantitative formation evaluation, especially for evaluation of formation resistivity anisotropy at all apparent dip angles, has not heretofore been used.
Interpretation of wireline 3D induction measurements is based on one dimensional parametric inversion. See, e.g., Wang, H., Barber, T., Rosthal, R., Tabanou, J., Anderson, B., and Habashy, T., Fast and rigorous inversion of triaxial induction logging data to determine formation resistivity anisotropy, bed boundary position, relative dip and azimuth angles, presented at the 2003 SEG Annual Meeting, October 27-30, Dallas, Tex. However, there is no such inversion procedure available for multi-axial LWD measurements, where ideally the resistivity anisotropy interpretation would be done essentially in real-time during the drilling of the wellbore.
U.S. Pat. No. 6,998,844, issued to Omeragic et al and assigned to the assignee of the present invention, describes making electromagnetic propagation measurements using transverse and tilted magnetic dipole antennas (“transverse” and “tilted” in the present context means with reference to the longitudinal axis of the well logging instrument). Such antennas are used in the PERISCOPE instrument described above. The '844 patent also describes a method for removing the “invasion” effect (effect of change in resistivity of formations proximate the wellbore wall by displacement of native fluid in the pore spaces with liquid phase of the drilling fluid) and shoulder-bed effect (effects of axially adjacent formations to the one under evaluation) from the anisotropy measurement, based on model-based parametric inversion. U.S. Pat. No. 6,594,584, issued to Omeragic et al and also assigned to the assignee of the present invention discloses a distance-to boundary parametric inversion that includes anisotropy inversion from an interval (longitudinally along the wellbore) of electromagnetic measurement data.
There continues to be a need for determination of formation resistivity, resistivity anisotropy and formation bedding attitude (“dip”) from electromagnetic measurements made during the drilling of a wellbore.